This invention relates to thin or low-profile antennas, and particularly to thin or low-profile antennas having good radiation capacities for receiving and/or sending radio frequency signals at a low angle to the major surface of the antenna.
The standard telecommunications (e.g. cellular telephone) antenna seen on the exteriors of automobiles today is a vertical antenna. This antenna presents a number of difficulties. First, it is not suitable for use with satellite communication services including current GPS and direct satellite broadcast services since those services may rely on satellites positioned most or less overhead where the vertical antenna lacks sensitivity. Second, future telecommunication systems will put more demands upon antennas. If vertical antennas were used to try to meet this demand, a number of antennas would be installed on the roof of a vehicle and as the desired performance of the antennas increased so would their numberxe2x80x94a forest of antennas could result. Third, these vertical antennas are (i) unsightly, (ii) subject to increased risk of breakage and damage and (iii) non-aerodynamic, particularly as their numbers increase. Forth, vertical antennas are effective only with vertically polarized radio frequency signals. A modern antenna needs to be able to handle both vertical and non-vertical emissionsxe2x80x94satellite emissions are apt to be circularly polarized.
The ideal antenna for a vehicle, such as an automobile, would be an antenna which:
(1) has a very small profile (so that it does not protrude in any significant way from the surface of the vehicle in which it is mounted);
(2) can handle radio frequency signals of different polarizations; and
(3) has both acceptable low angle (to the major surface of the antenna) efficiency and at the same time can handle communications with satellites positioned overhead.
The present invention has advantages for producing low-angle radiation from a low-profile antenna. The antenna may be horizontally mounted and, indeed, it may be conveniently mounted on or in the exterior surfaces of vehicles such as automobiles, trucks, trains and aircraft. With the future introduction of high-speed third-generation wireless data communication systems, such as third generation cellular systems, there will be a need for antennas that have appreciable gain near the horizon, since these systems will be primarily involving communications with ground base stations. Furthermore, for satellite-based direct broadcast radio and two-way communication systems there is also a need for the antenna to have significant gain at angles as low as 30 degrees from the horizon or lower as well as have the capability to serve satellites which are positioned overhead.
For mobile users in vehicles, one possible location for such an antenna is in the roof of the vehicle over the occupant area, which provides a broad area that can accommodate multiple antennas. However this can involve radiating at a low angle across a large metal surface, which is difficult particularly for horizontal or circular polarizations. Historically, the only way to produce significant antenna gain near the horizon is to provide an antenna with significant vertical heightxe2x80x94usually a large fraction of a wavelength depending on the antenna design. The use of a tall vertical antenna reduces the aerodynamic performance of the vehicle and is often quite undesirable for aesthetic styling purposes.
The present invention provides a good alternative, because it provides a specific method for producing low-angle radiation for horizontal, vertical, and circular polarizations while at the same time maintaining a low-profile shape. Antennas using this technique typically have a vertical height of much less than one-quarter wavelength.
The prior art includes the following patent application owned by UCLA: D. Sievenpiper and E. Yablonovitch, xe2x80x9cCircuit and Method for Eliminating Surface Currents on Metalsxe2x80x9d U.S. provisional patent application serial No. 60/079953, filed Mar. 30, 1998 and corresponding PCT application PCT/US99/06884, published as WO99/50929 on Oct. 7, 1999, the disclosures of which are hereby incorporated herein by reference.
Related patent applications include the following U.S. Patent Applications all of which are hereby incorporated hereby by reference:
1) D. Sievenpiper, R. Harvey, G. Tangonan, R. Y. Loo, J. Schaffner, xe2x80x9cA Tunable Impedance Surfacexe2x80x9d, U.S. Ser. No. 09/537,923, filed Mar. 29, 2000.
2) D. Sievenpiper, T. Y. Hsu, S. T. Wu, D. M. Pepper, xe2x80x9cAn Electronically Tunable Reflectorxe2x80x9d, U.S. Ser. No. 09/537,922, filed Mar. 29, 2000;
3) D. Sievenpiper, G. Tangonan, R. Loo, J. Schaffner, xe2x80x9cA Tunable Impedance Surfacexe2x80x9d, U.S. Ser. No. 09/589,859, filed Jun. 8, 2000.
4) D. Sievenpiper, J. J. Lee, S. Livingston, xe2x80x9cAn End-Fire Antenna or Array on a Surface with Tunable Impedancexe2x80x9d, U.S. Ser. No. 09/537,921, filed Mar. 29, 2000;
5) D. Sievenpiper, J. Schaffner, xe2x80x9cA Textured Surface Having High Electromagnetic Impedance in Multiple Frequency Bandsxe2x80x9d, U.S. Ser. No. 09/713,119, filed Nov. 14, 2000.
6) D. Sievenpiper, H. P. Hsu, xe2x80x9cA Polarization Converting Reflectorxe2x80x9d, U.S. Ser. No. 09/520,503, filed Mar. 8, 2000;
7) D. Sievenpiper, H. P. Hsu, G. Tangonan, xe2x80x9cPlanar Antenna with Switched Beam Diversity for Interference Reduction in Mobile Environmentxe2x80x9d, U.S. patent application Ser. No. 09/525,831 filed Mar. 15, 2000.
8) D. Sievenpiper, xe2x80x9cA Vivaldi Cloverleaf Antennaxe2x80x9d, U.S. Ser. No. 09/525,832, filed Mar. 15, 2000.
9) D. Sievenpiper, A. Schmitz, J. Schaffner, G. Tangonan, T. Y. Hsu, R. Y. Loo, R. S. Miles, xe2x80x9cA Low-Cost HDMI-D Packaging Method for Integrating a Novel and Efficient Reconfigurable Antenna ces and High Impedance Surfacexe2x80x9d, U.S. Ser. No. 09906035 filed Jul. 13, 2001.
10) J. Schaffner, D. Sievenpiper, J. Lynch, R. Y. Loo, xe2x80x9cA Reconfigurable Antenna for Multiple-Band, Beam Switching Operationxe2x80x9d, U.S. Ser. No. 09/629,681, filed Aug. 1, 2000.
11) D. Sievenpiper, H. P. Hsu, J. Schaffner, G. Tangonan, xe2x80x9cLow-profile, Multi-antenna Module, and a Method of Integration into Vehiclexe2x80x9d, U.S. Ser. No. 09/905,757, filed on the same date as the present application.
As is briefly discussed above, with the advent of broadband wireless communication systems, there is a need for antennas that can meet stringent performance criteria. At the same time, vehicle styling and/or aerodynamic requirements prohibit the use of unsightly xe2x80x9cantenna farms or forestsxe2x80x9d with multiple vertical antennas protruding from the surface of a vehicle. Hence, new antennas must not only have increased functionality to handle modern broadband wireless communication systems, but must also have a low-profile and should be conformable to the shape of the vehicle. In many situations, these two requirements are in direct conflict. For example, in modern communications systems, antennas should be able to handle low-angle radiation. For terrestrial systems, in which a mobile user is communicating with one or more base stations, the user must radiate energy at or near the horizon and typically in the microwave frequencies. For a handset such as a cellular phone, this is accomplished easily with a vertical whip antenna, which produces a nearly omnidirectional radiation pattern. For vehicle antennas, which are typically mounted on the top of the roof in order to obtain unobstructed coverage of all azimuthal angles, the presence of a large metal ground plane complicates the situation. In this case, a vertical monopole antenna is still sufficient for vertical polarization. However, as more functionality is added to the antenna, such as diversity combining, or beamforming, multiple monopole antennas then are needed, resulting in an unsightly and unaerodynamic xe2x80x9cantenna farm or forestxe2x80x9d. Furthermore, if horizontal polarization or circular polarization is required, the vertical monopole antenna is not a viable option.
Other antennas exist which have a low-profile and are capable of generating any desired polarization. The most common example of such an antenna is the patch antenna which consists of a small flat metal shape separated from a ground plane by a thin dielectric layer. One disadvantage of the patch antenna is that it cannot radiate effectively at low angles, rather it radiates the bulk of its energy in a direction normal to the ground plane. This is true of many low-profile antennas, particularly those having horizontal polarization or circular polarization (which consists of equal parts of horizontal and vertical polarization which are out of phase by 90 degrees). The reason for this is that a conductive ground plane does not allow the presence of a tangential electric field at its surface. In order to radiate at low angles (to the ground plane), the antenna must be able to generate a wave that skims across the metal surface, parallel or nearly parallel to the metal itself. This may be thought of as a kind of a surface wave. Vertically polarized radiation may occur at the horizon if the ground plane supports transverse-magnetic (TM) surface waves. Conversely, horizontally polarized radiation may occur at low angles if the ground plane supports transverse-electric (TE) surface waves. The fact that a flat metal surface does not support the propagation of TE surface waves is consistent with the fact that low-angle radiation with horizontal polarization is impossible from a conventional low-profile antenna. It is only when a conventional antenna is elevated a significant distance from its ground plane that it can effectively radiate at low angles. From another point of view, the effective image of the antenna in the ground plane cancels the radiation from the antenna in the case of horizontal polarization.
One possible solution to this problem is to use a high-impedance (Hi-Z) surface as the ground plane. The high-impedance surface consists of a flat sheet of metal covered by a two-dimensional array of resonators that can be analyzed as LC circuits, in which the resonance frequency is determined by the sheet inductance L and the sheet capacitance C. Near its resonance frequency, the surface provides an electromagnetic boundary condition that is the opposite of an ordinary metal surface, and it behaves as an effective magnetic conductor. The reader is directed to the other co-pending applications noted above and to PCT publication WO99/50929 also noted above for additional information relating to high impedance (Hi-Z) surfaces. This may seem to be a good choice for producing low-angle, horizontally polarized radiation. However, in its conventional form, the high-impedance surface fails at this task just as the metal surface does. The reason for this is that near its resonance frequency, the high-impedance surface suppresses both TM and TE surface waves. Thus, an antenna on such a surface cannot generate low-angle radiation of either polarization, and instead radiates most of its energy normal to the major surface thereof.
However, in connection with the present invention, it has been determined that it is possible to build an antenna that generates both horizontal and vertical polarization at low angles through an understanding of two non-obvious observations:
(1) High-impedance surfaces support leaky TE surface waves at frequencies above their resonance frequency, which can be used to generate horizontally polarized low-angle radiation.; and
(2) These leaky TE surface waves can also couple to TM surface waves on a nearby metal surface to generate vertically polarized radiation at low angles. Thus, with the proper combination of high-impedance surface and low-impedance surface (metal), one can build an antenna that produces low-angle radiation of both horizontal and vertical polarizations.
One common antenna that is known in the prior art is the patch antenna, shown in FIGS. 1a and 1b. It consists of a small shape of metal 10, usually circular or rectangular, that lies parallel to a larger metal sheet that serves as its ground plane 14. It is separated from this ground plane by a thin insulator 12 that is typically much less than one-quarter wavelength thick. It is often fed by a coaxial line 16, as shown in these figures, with the center conductor thereof being coupled to a feed point 18 on the patch antenna""s active element 10; however, other kinds of feeds may be used, such as a microstrip feed, or an aperture coupled feed. The length of the patch is generally equal to xc2xdn, where n is the refractive index of the substrate material. Thus, for a substrate with a higher refractive index (or dielectric constant), a patch antenna can be made shorter. It acts as a half-wavelength resonant cavity, and it radiates within a narrow band around its resonant frequency. A typical radiation pattern for this prior art patch antenna is shown in FIG. 2. In FIG. 2, the E-plane is shown in a thin line while the H-plane is shown in a thick line. In both planes, the radiation intensity tends towards zero near the horizon and is maximal normal to the surface. In is embodiment, the patch antenna 10 was mounted over a square metal ground plane 18 measuring twenty four inches (61 cm) on a side.
While the patch antenna is low-profile and suitable for mounting on the exterior of a vehicle, it is not very effective for producing low-angle radiation, particularly in a horizontal polarization. The reason for this is the presence of the metal ground plane, which suppresses the propagation of electromagnetic waves that have their electric field oriented parallel to the metal surface.
One alternative to the prior art patch antenna of FIGS. 1a and 1b is the high-impedance (Hi-Z) surface 30, shown in FIGS. 3a and 3b, with a suitable antenna element 10 disposed thereon. The Hi-Z surface 30 consists of a metal surface or ground plane 14 covered with a two-dimensional lattice of metal resonant elements 20, which typically resemble xe2x80x9cthumbtacksxe2x80x9d protruding from the metal ground plane 14. Near the resonance frequency, the Hi-Z surface 30 provides a boundary condition that is opposite to that of a flat conductive surface. This allows antenna elements, such as antenna element 10 depicted by FIGS. 3a and 3b, to lie directly adjacent to the Hi-Z surface 30 without being shorted out, resulting in antennas that are much less than one-quarter wavelength thick, yet radiate effectively within a particular frequency band. An example of such an antenna element is the horizontal bent-wire antenna 10 shown in FIGS. 3a and 3b, but other types of antennas may be used instead, including one or more patch antennas. A bent-wire antenna is typically one-third to one-half wavelength long and consists of a wire that extends from the back of the surface (the wire which extends may be simply the center conductor of a coaxial cable) to the front, where it is bent over parallel to the surface. More than one antenna could be used on the surface and, ordinarily, more than one antenna would be used. This would be done to provide a particular desired radiation pattern, or perhaps several different radiation patterns, so that one could switch among the patterns. The operating frequency of antenna 10 is determined by the properties of the Hi-Z surface 30, in particular its sheet capacitance and sheet inductance, as well as by the size of antenna 10. For a surface with sheet capacitance C, and sheet inductance L, the resonance frequency will be   ω  =      1          LC      
and the bandwidth will be   BW  =                              L          /          C                    377        .  
The antenna element 10 is depicted as being coupled to a coaxial line 16, as shown in FIG. 3b, with the center conductor thereof being coupled to a feed point 18 on the antenna element 10; however other kinds of feeds may be used, such as a microstrip feed or an aperture coupled feed. The metal resonant element(s) 20 which would otherwise be in the way of the feed point 18 is (are) omitted in this embodiment so that the feed point 18 is not shorted to the ground plane 14. Alternatively, the feed point 18 can be located in the regions between resonant elements 20. The metal resonant elements would in an actual embodiment be much smaller than that depicted in FIGS. 3a and 3b and are depicted enlarged in these figures for ease of illustration. The size of the elements 20 is largely governed by the frequency (and bandwidth) at which the Hi-Z surface is to operate as governed by the aforementioned equations.
Near the resonance frequency, the Hi-Z surface 30 has the additional property that it suppresses the propagation of surface waves. In many antenna applications this is a desirable property, because the antenna will not excite unwanted currents on or in nearby metal objects. This can be particularly important for electromagnetic interference (EMI) reduction, and electromagnetic compatibility (EMC) concerns, in which it is desirable to minimize the amount of coupling between nearby electronic devices or other nearby antennas. In order to accomplish this goal, a conventional Hi-Z surface is employed under or around an antenna 10 and the antenna 10 is operated at or near the resonance frequency of the Hi-Z surface.
In one aspect the present invention provides a technique to produce an electrically thin antenna that has increased low-angle radiation in comparison with other antennas having a similar profile. It does this by using an area of a high-impedance surface which is encompassed by a larger region of metal surface. Producing improved low-angle radiation is accomplished through the excitation of a tangential electric field on the high-impedance surface, as well as leaky transverse-electric surface waves. Such fields and surface waves cannot normally occur on an ordinary metal surface. The tangential electric field on the high-impedance region excites a transverse-magnetic surface wave on the surrounding metal surface which gives improved low-angle radiation in the E-plane of the antenna. The leaky transverse-electric surface waves provide improved radiation in the H-plane of the antenna.
One of the novel features of the present invention is based upon the use of a high-impedance (Hi-Z) surface outside its usual operating region (which is the surface wave band gap) and instead operating in a different region (the transverse-electric surface wave region). Instead of trying to suppress surface waves, as is usually done with conventional Hi-Z surfaces, the present invention, according to one aspect thereof, takes advantage of a Hi-Z surface by using it not in a frequency region where it suppresses surface waves, but in a frequency region where it supports leaky TE surface waves in order to achieve improved low angle radiation. Thus, in one aspect the present invention includes (i) the use of a high-impedance surface (which may by itself be of a conventional design) at frequencies outside of its usual operating mode, and (ii) a geometry consisting of a combination of high-impedance surface and low-impedance (for example, metal) surface which are designed to achieve the desired low-angle radiation pattern.